Consider the following epistemological argument against Naturalism (as defined below):
- If Naturalism is true, then all factual knowledge (i.e., knowledge of facts) is acquired empirically.
- Knowledge of necessary facts cannot be acquired empirically.
- We have some knowledge of necessary facts.
- Therefore, Naturalism is not true.
The insights behind the argument aren’t original to me, but the formulation is mine. Before I defend the three premises, let me clarify the key terms used in the argument.
Naturalism refers to the ontological thesis that only natural things exist, that is, things that exist spatiotemporally and can be described according to our best physical theories. On this definition, Naturalism is roughly equivalent to physicalism, the view that the fundamental ‘stuff’ of reality is physical and whatever exists can be accounted for (in principle) in terms of physical reality (physical particles, physical forces, etc.).1
Factual knowledge refers to knowledge of facts about the world; specifically, facts about an external world that exists independently of our minds. Factual knowledge is distinct from (1) analytical knowledge (i.e., knowledge of logical or conceptual truths, such as that a triangle is a polygon) and (2) knowledge of internal mental states (e.g., that I am currently experiencing pain). A simple example of factual knowledge would be that there are, at this present moment, more than two turtles in Turtle Pond.
Empirically means by way of sense experience or observation, that is, by means of our sensory organs (the standard five senses or any others we might have that operate on a similar basis).
Necessary facts are facts about what must be the case, as opposed to what merely is the case or could be the case. To know a necessary fact is to know not merely that something actually is the case, but also that it could not possibly have failed to be the case. Knowledge of necessary facts is a species of factual knowledge (as defined above).
Now, back to the argument. I think it’s fairly clear that the argument is logically valid: the conclusion follows from the three premises. If premises 2 and 3 are true, it follows that some factual knowledge is not acquired empirically, in which case — by modus tollens from premise 1 — Naturalism is not true.
So why think that the premises are true? Consider each in turn.
Premise 1: If Naturalism is true, then all human knowledge is stored in the brain by way of physical structures and properties. Furthermore, the only way for factual knowledge (i.e., knowledge of how things are in the external world) to get into the brain is by way of physical sensory organs, or, more loosely, physical ‘input devices’. Even if we could directly ‘input’ information to the brain (e.g., via something like Elon Musk’s Neuralink technology) that would still be analogous to our ‘natural’ sensory organs and thus a kind of empirical (observational) knowledge. All this to say, given the Naturalist’s view that we are purely physical organisms and entirely the product of physical processes (the grand evolutionary story of human origins), it’s very hard to see how we could have a priori factual knowledge.
Objection to Premise 1: “We could have innate factual knowledge that has been ‘hardwired’ into our brains by way of evolutionary processes. Perhaps we all know some fact F, not because we have personally observed F to be the case, but because our ancestors frequently observed it, and by natural processes operating under evolutionary pressure their observational beliefs became permanently encoded in the human brain, such that knowledge of F is now innate. On this account, I know F, but not by way of sense experience.”
Response: There are several problems with this proposal, but the main one is this: it still requires someone to know F empirically. On the evolutionary account, my knowing F might be derivative of my ancestors’ knowing F, but it can’t be derivative all the way down. Innate knowledge in present-day humans would ultimately depend on empirical knowledge in our ancestors. On the Naturalist view, the only way for factual information to “get into the human brain” — whether individually or collectively — is via physical sensory organs.2 So it’s still true that given Naturalism, all factual knowledge is ultimately acquired empirically. (Adjust premises 1 and 2 accordingly.)
Premise 2: It’s widely recognized (at least since David Hume) that empirical observations cannot in principle deliver knowledge of necessary facts. Empirical observations can tell us only how things are, not how things must be. Cast in the terminology of possible worlds, our senses only give us access to this possible world — to the actual world — not to all possible worlds. (A fact is a necessary fact just in case it is a fact in all possible worlds.) From a purely empirical perspective, a necessary fact is indistinguishable from a contingent-but-constant fact (i.e., a fact that could have been otherwise, but happens never to be otherwise in the actual world).
Premise 3: All that’s needed to support this premise is one example of a necessary fact that we know. Here is one: No physical object is entirely red and entirely green at the same time. Here’s another one: No water molecule contains a carbon atom. Here’s a third: If there’s one stone in the bucket, and I add another stone, there will be two stones in the bucket. Here’s a fourth: It is morally wrong to torture infants for fun. These are not merely facts, but necessary facts: facts that could not be (or have been) otherwise. Anyone who rejects premise 3 will have to say one of three things about these examples: (1) we don’t know them to be necessary facts; (2) they aren’t necessary facts (i.e., they’re merely contingent facts); (3) they aren’t facts at all. Each option carries a cost.
In sum, I think there are good reasons to affirm all three premises, and since the conclusion follows from the premises, I think the argument provides good reason to believe that Naturalism is not true.
Of course, if you want to hold on to Naturalism, you’ll have to reject at least one of the premises. But which one, and why? (If you take issue with my definition of Naturalism, that’s beside the point; I’m offering an argument against Naturalism according to my definition, which is a fairly conventional definition.)
One possible way out for the Naturalist is to “go Kantian” and adopt some form of transcendental idealism, but I can’t imagine many Naturalists will be rushing to take that exit.
- The physicalism could be reductive or non-reductive; I don’t think it makes a difference to the argument. Some self-professed naturalists hold to a more liberal ontology, e.g., allowing for sets or abstract mathematical objects. Whether those more liberal versions of naturalism are vulnerable to this argument is an open question that I won’t address here. ↩
- By analogy, think of computers that build (‘beget’) other computers. The descendant computers could be created with built-in (‘innate’) information that comes from the ancestor computers rather than their own input devices, but at some point back down the line, some ancestor computer had to get its information from an external source. ↩